1. Field of the Invention
The present invention relates to an apparatus and method for searching for a cell of a mobile communication system, and more particularly, to an apparatus and method for searching for a cell by identifying a preamble index corresponding to a signal received from a radio access station (RAS) in a mobile communication system supporting orthogonal frequency division multiple access (OFDMA) mode.
2. Description of Related Art
A “cell searching apparatus” and a “cell searching method” as used in the present specification may mean all apparatuses and methods which can search for a cell or a sector. In this instance, the cell or the sector is interpreted as a regional area corresponding to each RAS of a mobile communication system. The “cell searching apparatus” may either operate as a stand-alone device or as an embedded part of a mobile communication system including portable devices and/or measuring equipment, etc.
As known to those skilled in the related art, data are transmitted by a frame unit in an OFDMA mobile communication system. FIG. 1 illustrates an example of an OFDMA time division duplex (TDD) frame structure complying with a Mobile Broadband (WiBro) standard. The frame structure illustrated in FIG. 1 follows an Institute of Electrical and Electronics Engineers (IEEE) 802.16d/e standard. FIG. 1 illustrates a downlink (DL) frame from a connection between a mobile base station, i.e., an RAS, and a portable device, i.e., a portable subscriber station (PSS). Also, the DL frame includes a preamble, a DL subframe and an uplink (UL) subframe. Hereinafter, a “preamble” as used in the present specification means a DL preamble included in a DL frame.
Referring to FIG. 1, a preamble is allocated to a first symbol of a DL frame. The allocated preamble is utilized for frame synchronization and cell partition. In association with the technical field where the present invention is applied, the preamble represents cell identification number (ID) and segment number information which can identify a cell of a mobile communication system. Accordingly, the portable device may identify the cell by using the preamble and may set up a connection with an RAS corresponding to the identified cell.
FIG. 2 is a diagram illustrating a segment-wise preamble transmission structure of an OFDM/OFDMA mobile communication system. As illustrated in the figure, each guard band is provided on a left side and a right side of a plurality of subcarriers to reduce an interference of a neighboring frequency band, and a direct current (DC) subcarrier, which is a null subcarrier, is provided. Also, as illustrated in the figure, preamble subcarriers for each segment are arranged at a predetermined interval, for example, three subcarrier intervals, in FIG. 2. An RAS corresponding to each cell may transmit a preamble signal containing preamble information corresponding to a given cell ID by using a segment allocated to the RAS. Accordingly, a portable device may identify the preamble index corresponding to a signal received from the RAS. A preamble signal is used as an example of the received signal in the present specification. Also, the portable device may identify a cell ID of the RAS and a segment number given to the RAS from the received signal.
In the present specification, the description is limited to only when a total number of segments is three. However, it will be apparent to those skilled in the related art that an application range of the cell searching apparatus and method described in the present specification is not limited thereto, but applicable according to a type of a mobile communication system and a standard thereof.
A cell searching apparatus is an apparatus necessary for setting up a connection between a portable device and an RAS, and stably transmitting/receiving data therebetween. At every instance of a physical movement of the portable device or a change of wireless channel state, the portable device has to re establish a connection with the RAS. For this, a cell containing the portable device needs to be searched for. In particular, when a handover occurs frequently according to a movement of the portable device between cells or sectors, a cell searching performance of the portable device becomes a very important factor to determine the quality of mobile data communication.
As illustrated in FIG. 2, a preamble received in a portable device may include a plurality of preamble signals which are transmitted from a plurality of RASs. Accordingly, the portable device may determine the strongest preamble signal from among the received preamble signals and set up a connection with an RAS which can transmit/receive data in the strongest signal level, among a plurality of neighboring cells as well as the cell containing the portable device.
A conventional cell searching apparatus performs a correlation in a frequency domain. As is known, an RAS of an OFDMA system converts a frequency domain preamble signal containing data represented by a plurality of subcarriers to a time domain signal via an Inverse Fast Fourier Transform (IFFT) module and transmits the converted signal. Accordingly, the conventional cell searching apparatus initially converts a received preamble signal to a frequency domain and subsequently correlates the converted preamble signal with preamble sequences stored in a portable device, in a frequency domain.
FIG. 3 is a diagram illustrating an example of the conventional cell searching apparatus which performs a correlation in a frequency domain, as described above. A cell searching method using the cell searching apparatus illustrated in the figure is as follows. Initially, a correlation value between y(k) and Pd(k) is calculated, in which y(k) is a received signal in a frequency domain via a Fast Fourier Transform (FFT) module 310 and Pd(k) is a preamble sequence stored in a cell searching apparatus. A correlation module for obtaining the correlation value may include a delay module 320, a complex conjugate module 330, a multiplier and a summator 340. Each correlation value is calculated with respect to D pre-stored preamble sequences, and a preamble sequence having a maximum correlation value may be obtained therefrom. When a preamble index which is an index value corresponding to the obtained preamble sequence is didentified, the process is represented as Equation 1 below,
                              d          identified                =                                            arg              ⁢                                                          ⁢              max                        d                    ⁢                      {                                          ∑                                  k                  =                  0                                                  K                  -                  1                                            ⁢                                                y                  ⁡                                      (                    k                    )                                                  ⁢                                                      P                    d                    *                                    ⁡                                      (                    k                    )                                                                        }                                              [                  Equation          ⁢                                          ⁢          1                ]            
In this instance, K indicates a number of subcarriers constructing a single preamble sequence, and d an index of each candidate preamble sequence. As an example, K may have a value of ‘284’ which is obtained by dividing ‘852’ subcarriers by three, a total number of segments. In this instance, ‘852’ subcarriers are obtained by subtracting ‘172’ guard band subcarriers from ‘1024’ total subcarriers,
The conventional cell searching apparatus as described above requires detecting accurate frame boundary timing and a subcarrier frequency offset for a normal operation. An error which occurs during an initial frame boundary timing detection process makes FFT timing information inaccurate. A phase offset is generated due to the inaccurate FFT timing information. The generated phase offset deteriorates a general searching performance. In particular, when a portable device is positioned in a cell boundary, the conventional cell searching apparatus may not produce an accurate search result.
Also, when reflecting the influence of a subcarrier frequency offset, the conventional cell searching apparatus may not identify a segment number from a preamble signal. Accordingly, the load on the portable device increases in proportion to a total number of segments, for it requires additional hypothesis test for each case of segment number. For the initial carrier frequency offset estimation case, the segment number's uncertainty problem before cell searching process requires three times more hypothesis tests (with three segment number) per each preamble's correlation operation. This kind of operation mandates heavy computation load and implementation complexities which need to be avoided for low power and low cost implementation option at the PSS side.
Accordingly, a new technology which can quickly and accurately search for a cell from a received preamble signal in a mobile communication system by using less resource as compared to the conventional art.